Process for conditioning sinter draft for electrostatic precipitation of particulate material therefrom

ABSTRACT

There is provided an improved traveling grate sintering process for treating solid mineral matter such as iron ore and utilizing a sinter draft system which recycles a part of the total draft, minimizes the exhaust draft effluent, and reduces the costs of draft treatment to meet environmental air quality standards. The system contemplated herein recycles relatively cool draft from the initial windboxes adjacent the traveling grate to the hood or hoods downstream of the path of grate travel to significantly decrease the concentration of hydrocarbons in the sinter exhaust gases and to produce a hotter, more humid exhaust for subsequent processing to remove pollutants, particularly with respect to the electrostatic treatment of the exhaust draft to remove solid particulate material. At the same time a sinter product comparable in quality to that produced by the basic downdraft sintering system is obtained without substantial change in rates of production.

1 1 Sept. 30, 1975 United States Patent [19] Ban 1 1 PROCESS FORCONDITIONING SINTER DRAFT FOR ELECTROSTATIC PRECIPITATION OF PARTICULATEMATERIAL THEREFROM [75] Inventor: Thomas E. Ban, South Euclid. Ohio [73]Assignee: McDowell-Wellman Engineering Company. Cleveland, Ohio [22]Filed: Nov. 6. 1972 [21] Appl. No.: 303,953

Related U.S. Application Data [63] Continuation-impart of Ser. No.174,714. Aug. 25.

1971. abandoned.

[52] U.S. C1. 432/16; 75/5; 432/18; 432/72 [51] Int. C1. ..1F27B 15/12[58] Field of Search 75/5: 432/16. 18. 72

[561 References Cited UNITED STATES PATENTS 1.836.176 12/1931 Klcncke266/21 2.006.368 7/1935 Reid...... 75/5 X 2.861.881 11/1958 Phelps 75/5X 3.043.677 7/1962 Monaghan 75/5 3.205.064 9/1965 Chang 75/5 3.257.1956/1966 Schwarz 75/5 3.264.091 8/1966 Ban 75/4 X 3.311.465 3/1967 Ban eta1 75/5 Primary Examiner-John J. Camby Attorney. Agent. or FirmMcNenny,Farrington, Pearne & Gordon [57] ABSTRACT There is provided an improvedtraveling grate sintering process for treating solid mineral matter suchas iron ore and utilizing a sinter draft system which recycles a part ofthe total draft, minimizes the exhaust draft effluent. and reduces thecosts of draft treatment to meet environmental air quality standards.The system contemplated herein recycles relatively cool draft from theinitial windboxes adjacent the traveling grate to the hood or hoodsdownstream of the path of grate travel to significantly decrease theconcentration of hydrocarbons in the sinter exhaust gases and to producea hotter. more humid exhaust for subsequent processing to removepollutants. particularly with respect to the electrostatic treatment ofthe exhaust draft to remove solid particulate material. At the same timea sinter product comparable in quality to that produced by the basicdowndraft sintering system is obtained without substantial change inrates of production.

10 Claims, 1 Drawing Figure COLD fETCI (ZE hard? US. Patent Sept.30,1975

PROCESS FOR CONDITIONING SINTER DRAFT FOR ELECTROSTATIC PRECIPITATION OFPARTICULATE MATERIAL TI-IEREFROM RELATED APPLICATION This application isa continuation-in-part of my copending application Ser. No. 174,714filed Aug. 25, 1971, now abandoned.

BACKGROUND OF THE INVENTION AND PRIOR ART The present invention relatesto an improved sintering process. Generally speaking, the well-knownprocess of sintering has heretofore comprised submitting a particulatemetallic oxide, e.g. iron ore or iron oxide (as from basic oxygenfurnace fume), in a burden on a moving grate to transversely moving hotoxidizing gases to fuse particles of the ore together to form anagglomerated mass. The agglomerated mass may then be used in a knownmanner in iron-producing equipment, e.g. a blast furnace. Frequently,the ores contained naturally or were made to contain a combustiblematerial, e.g. coke or coal, and a flux, e.g. limestone and/or dolomite,and a burden of a granular mixture with or without added moisturedeposited upon a traveling grate. In the initial stages of the movementof the burden along its predetermined path on the traveling grate, theburden was ignited by passing under gas torches or oil burners wherebyignition of combustibles and a flame front were established in theburden. Thereafter, by means of fans, ambient air was drawn downwardlythrough the burden as it moved along the path causing the flame front tomove downwardly through the burden toward the grates. The temperature ofthe burden by this process was raised along the flame front toapproximately 2500F. (lower or higher depending upon the ore), wherebythe individual particles became fused into a solid foraminous mass. Asthe hot sinter was discharged from the end of the traveling grate, itwas broken into large chunks. The residence time on the traveling gratewas sufficient to cause the fusion to occur substantially through thedepth of the burden. The gases which were exhausted from the lower sideof the burden when downdraft sintering was employed were generallyexhausted to the atmosphere. These gases in addition to containingentrained and fumed solid particulate material also contained unburnedhydrocarbons, sulfur compounds, and oxides of carbon.

Recent efforts to control atmospheric pollution have necessitatedreconsideration of the methods by which sinter exhaust gases arehandled. To remove hydrocarbons and lower oxides of carbon, afterburneror incinerator means have been utilized to ignite the combustiblecomponents of the exhaust sinter gases and convert them to harmlessgases, e.g. carbon dioxide and water vapor. However, afterburner andincinerator means typically employ hydrocarbon fuels which can result inan increase in the concentration of hydrocarbons in the exhaust gases.Entrained sulfur or lower sulfur oxides are also converted to higheroxides which may be removed by scrubbing means in a known manner. Thelarge quantity of solid particulatematerial entrained in such exhaustsinter gases typically is passed through electrostatic means whicheffect precipitation of such particulate material in a known manner.Unfortunately, conventional sintering practice is such that the moisturecontent and temperature of the exhaust gases are not optimal forcleaning by electrostatic means. Thus, in order to improve thecombustion and formation of sinter, particularly in the case of ironore, the mixture of iron ore and carbonaceous material and optionally afluxing material, e.g. limestone and/or dolomite, has usually beenmoistened with up to 6 to 14 percent by weight of water to form wetagglomerated masses of approximately the size of rice. This enables theuse of finely divided carbonaceous material and facilitates thedistribution thereof with respect to the iron oxide in a wetagglomerated mass. The use of water as a binder aid results, duringsintering, in considerable moisture in the gases traversing the burden.If the burden is too deep for the draft system employed as the gasesencounter the lower regions of the burden which are at a much lowertemperature, the moisture picked up at or near the flame front iscondensed by and saturates the lower regions of the burden. When theproblem becomes acute, a condition known as sogging out occurs and thepassage of the gases through the burden is greatly impeded. Hence it hasbeen desired to severely limit the depth of the burden or to minimizethe amount of moisture in the burden and in the gases traversingtherethrough in order to approach the capacity of the apparatus.However, when the moisture content of the gases is low, the resistivityof the gases and particles is high and the effectiveness ofelectrostatic precipitation means is greatly reduced. Also, thetemperature of the gases normally passing through the burden inconventional procedures and exiting from the lower side thereof is low(225 to 300F.). At both low moisture content and such relatively lowtemperature, the electrostatic precipitation characteristics aregenerally inferior.

Thus, conventional sintering of mineral oxidecontaining ores ascurrently practiced is confronted by a dilemma: If the moisture contentof the gases is raised within the system to improve the operation ofelectrostatic precipitation equipment, sogging out of deep sinter bedsis encountered. If moisture is added to the exhaust gases aftersintering, dew point condensation is reached which causes fouling andcorrosion of the apparatus. While the capacity can be cut by usingthinner beds or burdens to raise the moisture content of the exhaustgases, such cuts tend to be too severe for economical operation and themoisture pick-up per unit volume of gas because of the lowered quantityof water available in thinner beds is still below optimum forelectrostatic precipitation techniques. Also, the temperature of theexhaust sinter gas is below optimum.

It has now been found that relatively deep horizontally disposed burdensof sinterable particles, comprised ofa metal oxide, shale, or clay,optionally a flux, a combustible carbonaceous material, and water, on atraveling grate of either linear or circular confirguration, may besintcred economically by the process of the present invention, whichincludes the steps of igniting the burden and passing oxidizing gasesthrough the burden to generate a transversely moving heat front in theburden while moving the burden horizontally along a predetermined pathor length of travel. In accordance with the present invention, soggingout of the burden is avoided, pollutants in the exhaust gases areminimized, and the exhaust gases are conditioned for more effectiveelectrostatic precipitation of entrained particulate solids bycollecting moisture-laden gases exiting through the burden in a firstportion of the path and recycling at least a portion thereof through aburden in the subsequent portion of the path. The exhaust sinter gasesexiting from such subsequent portion are now found to possesssubstantially reduced hydrocarbon concentrations and more nearly optimumcharacteristics for treatment in electrostatic precipitation means.While recycling through sinter burdens is not per se new (see thepatents to Dwight US. Pat. Nos. 1,215,636 and 1,215,637 and to KlenckeUS. Pat. No. 1,836,176), the ores so processed were sulfide oresnaturally containing combustible sulfurous compounds, the sinteringoperation produced primarily sulfur dioxide and sublimed sulfur, butlittle or no hydrocarbon pollutants in the exhaust drafts, and there wasno utilization of moisture for draft treatment. Dry gases cannotproperly be electrostatically treated because the resistivity is toohigh. The technology with respect to sintering, on the other hand, hasindicated it to be contrary to good sintering practice to introducewater vapor into a hood in a sintering process (see Agglomeration byKnepper, Interscience Publishers, 1962, Pages 464 466). It is also shownthat the sinter output is decreased with an increase of the water vaporin the sintering air or gases.

The present process is, as indicated above, characterized by takingmoisture-laden gases from an initial portion of the path traveled by theburden and deliberately recycling these relatively cool gases,preferably, after first passing them through a primary cleaner, into ahood and through the moving sinter bed or burden at a succeeding portionof the path traveled by the burden. 1

Those gases exhausting from the succeeding portion of the bed. as hasbeen noted above, contain reduced hydrocarbon levels, have a highhumidity, i.e. from about 5 to percent water by volume, are at anincreased temperature (4()0 600F.), and are dustladen. The gases can bepassed directly to an electrostatic precipitator for precipitation ofthe dust particles, the increased moisture content and temperature ofthe gases making them more amenable to such treatment. Alternatively,the exhaust gases flowing from the bed can be passed through adust-collecting device to remove the coarser particles, then through anafterburner to burn any remaining hydrocarbons and a cooler to adjustthe temperature to that which is proper for electrostatic precipitation.When either of these methods of conditioning exhaust sinter gas has beenpracticed, both the moisture content or humidity of the gas and thetemperature thereof are more nearly optimum for proper operation ofelectrostatic precipitation equipment. No decrease in the rate of sinterproduction is observed, but numerous advantages, particularly from apollution standpoint, are achieved. As indicated, electrostaticprecipitation of solid particulate material is much more efficient.Also, it is apparent that the gases are sufficicntly conditioned forinitial charging to the electrostatic precipitator and final charging tothe afterburner. The recycling of the gases and the concomitantrepassage of hydrocarbon, carbon and carbon monoxide gases through thesinter bed achieves combustion of the hydrocarbon moieties andconversion of the carbon and carbon monoxide to harmless carbon dioxideand water, and raises the temperature above the usual 250F. average.Sulfur impurities are also oxidized and may be scrubbed from the exhaustgases by suitable scrubbing means. Thus, both the desired objectives ofeconomical sinter production and pollution control are accommodated bythe present process. Additionally, the volume of effluent gas is reducedby the amount recycled, aiding not only in respect of atmosphericpollution but also in cost of handling gases.

The present invention may be practiced while using the sintering machineonly for sintering and terminating the operation when the flame frontreaches the grates simultaneously with the discharge of the hot sinterfrom the machine. In such case the sinter may be subsequently cooled bya separate cooler machine using forced ambient air as the coolingmedium. If the sintering machine is extended to provide a cooling zonetherein beyond the sintering zone, this cooling zone does not give riseto entrained hydrocarbons within the gases because sintering iscompleted before the cooling zone, whereupon the charge becomescompletely depleted of hydrocarbonaceous material. However, if recycledraft containing entrained hydrocarbons is used as the cooling mediumand is directed through the hot sintered ore layer in the sinter zone,the hydrocarbons still present are reduced and provide an exhaust gasdepleted of entrained hydrocarbons.

BRIEF STATEMENT OF THE INVENTION Briefly stated, therefore, the presentinvention is in a process for sintering a burden of particles disposedon a traveling grate machine, such particles comprising any mineral orore, e.g. iron ore, iron oxide, shale, or clay, a combustiblecarbonaceous material, e.g. coal, coke, coke breeze, tar residues,lignite, etc., and water, and optionally a flux, by igniting the burdenand passing oxidizing gas, e.g. ambient air, through the ignited burden,preferably at a rate of from 1 5 lbs. of gas to each pound of burden, tomaintain combustion of the carbonaceous material and generate atransversely moving heat front in the burden while moving the burdenalong the traveling grate through a combustion zone. The improvement ofthe invention comprises collecting moisture-laden exhaust gases andentrained combustible matter (both gaseous and particulate solidmaterial) from an initial portion of the length of the combustion zone,recycling at least a portion thereof through the burden in a subsequentportion of the combustion zone to incinerate the entrained combustiblematter within the bed, and withdrawing exhaust gases from the bed alongthe subsequent portion of the com-. bustion zone, whereby the latterexhaust gases from the subsequent portion of the combustion zone have asubstantially reduced content of combustible matter and a substantiallyincreased moisture content and are, therefore, more amenable toeconomical further treatment for incinerating any residual combustiblematter therein in an afterburner or the like and for removal ofentrained particulate matter in an electrostatic precipitator or thelike.

Reference may again be had to Agglomeration by Knepper, supra, andparticularly to the paper contained therein entitled The ContinuousSintering Process-Research and Applications" by Ban, Czako, Thompson andVioletta (pp. 51 l to 536) for a discussion and understanding of theconcept of the flame front in a sinter bed. The usual verticalpropagation rate through the burden of the flame front is in the rangeof from about 0.25 inch to 2 inches per minute. Many parameters affectthe rate including oxygen content of the gas, fuel distribution, gasflow rate, moisture content of both the bed and the gases, porosity ofthe bed, etc. 1

BRIEF DESCRIPTION OF THE DRAWINGS In the annexed drawing there isdiagrammatically shown apparatus suitable for carrying out the improvedprocess of the present invention. The drawing shows a traveling gratemoving along a predetermined path in a direction from right to left andhaving 16 windboxes disposed in underlying relation to the grate, and agas hood disposed above the grate adjacent the terminal portion intowhich cold wet recycle gases are introduced. By cold in reference tothese gases is meant from 150 to 300F.

DETAILED DESCRIPTION OF THE PROCESS As indicated above, the burdentreated in accordance with this process is formed of particles of aplurality of materials. The solid materials are provided in the form ofgranules generally having a particle size in the range of one-fourthinch to fines. Thus, for example, granular iron ore, shale, or clay ismixed with a granular combustible carbonaceous material, e.g. coal,coke, coke breeze, lignite, returns, optionally flux material, e.g.limestone, etc. The proportions of these solid materials are generallyin the range of from 2 to percent by weight of coke or.coal or othersuch carbonaceous material on the dry basis. In order to properlyagglomerate the carbonaceous material and the metal oxide-containingmoiety, a liquid binder may be used. The liquid binder compositioncomprises mainly water although, for example, an asphalt emulsion may beused. The amount of moisture should be in the range of from 6 to 14percent by weight.

The particles from which the burden is made may be that which resultfrom combining the essential ingredients above mentioned and tumbling ina drum or by treating in a pelletizing apparatus such as that describedin US. Pat. No. 3,169,269 dated Feb. 16, 1965. The particles aredesirably shaped as kernels of rice and have approximately that size.This condition is known in the trade as rice-like texture.

A typical formulation on the dry basis for the moist or green burdenmaterial to be submitted to the sintering process of the processinvention is as follows:

Iron ore 3271 Mill scale 1071 Flue dust 27 Flux 23% Coke breeze 5%Returns 28% The dry materials are blended with water in any suit ableblending or pelletizing apparatus to a moisture content in the amount offrom 6 to 9 percent by weight, e.g. 6 percent. Any composition includinga metal oxide moiety, optionally a flux material, and a carbonaceousmaterial and moisture for sintering, numerous examples of which areknown to those skilled in the art,

' may be used in the practice of this invention, such composition notbeing critical to the process in any respect.

In one embodiment of the sintering process of the present invention themoving burden is ignited at the uppermost layers thereof to initiateburning of carbon or other combustible carbonaceous material containedtherein. Gases containing a sufficient amount of oxygen, e.g. air, arethereafter drawn downwardly through the bed to perpetuate the burningand cause vertical propagation of the heat front or flame frontdownwardly toward the grates. Where the metallic oxide moiety comprisesiron ore, the temperature within the bed in the heat front approaches2500F. Where clay is the metallic oxide-containing moiety, thetemperature reaches approximately 2200F. Under these conditions, thegranular mineral material is partially fused to form a foraminous cakeof agglomerated sintered material. The process is distinguished frommetallizing or reducing reactions because it is carried out in thepresence of an oxidizing draft as distinct from a reducing draft. As theflame front moves through the bed, hot gas immediately ahead thereofeffects volatilization or fuming of nonferrous metal oxides and avolatilization, distillation, or pyrollization of organic materials fromthe bed which are picked up by the moving gas and entrained therein. Ingeneral, organics are not burned in the draft stream below the flamefront because the temperature and the oxygen content of the gas at thispoint is insufficient for combustion. With reference to the drawing,these phenomena are occurring within the burden adjacent windboxes 1through 15. If sintering terminates adjacent windbox 15, the lastwindbox 16 is desirably used for cooling.

Referring now more particularly to the diagrammatic and schematicrepresentation in the annexed drawing, a moist burden of blendedparticulate material having a depth of from about 5 to about 15 inches(the latter being regulated by conventional gating means) is cast uponthe traveling grate (also of conventional design) adjacent windbox 1 andproceeds from there toward the discharge end as indicated in thedrawing. The longer the path, the more rapid the movement required forthe burden. While passing over the windboxes l and 2, the burden isignited in the upper layers by means of gas torches or burders (notshown). This causes preliminary drying of the surface of the particulateburden and elevates the temperature of the burden sufficiently that inthe portion of the path traveled by the burden above windboxes 3 6burning of the fuel moiety is establishcd. Air downwardly drawn throughthe burden by means of the recycle fan promotes such combustion. In theinitial portion of the total horizontal path traversed by the burden andindicated by windboxes 1 6, approximately 50 to percent of the water inthe burden is removed by the gases. The gases drawn downwardly throughthe burden by means of the recycle fan at a rate producing asubstantially violent draft, the rate being approximately from 0.5 2.5pounds of gas per pound of burden treated. The rate of flow of the gasesthrough the burden is in the range of from 300 standard cubic feet perminute per square foot of grate area (SCFM). Because of this relativelyhigh flow rate, fine particles of aggregate and decomposition productsthereof are mechanically entrained in the gas and pass through the bed.An initial dust collector 21 in series with the manifold connectingwindboxes l 6 aids in elimination of some of the coarser particles,which particles may comprise a portion of returns" in subsequent greenburden formulations. The temperature of the gases at this point is about200F.a relatively cold recycle gas. These gases are propelled by meansof a recycle fan 22 to a subsequent portion of the path traveled by theburden wherein the major portion of sintering actually occurs. By thetime the burden enters the region above windbox 7, the flame front hasbeen established due to the initiation of the combustion in the upperlayers of the burden. It continues its downward propagation through thebed as the bed proceeds from right to left as shown in the drawing.

The cold recycle gas contains a substantial quantity of moisture andalso contains unburned hydrocarbons, carbon monoxide, and dustparticles. The recycle fan introduces such cold recycle gas into a hood23 spanning windboxes 7 through in the embodiment shown. Under theinfluence of a sinter fan 24 in series with a header 26 joiningwindboxes 7 15 inclusive in the illustrated example the cold recycle gasis drawn downwardly through the sinter bed again at the rate which is onthe order of from 0.5 2.5 pounds of exhaust gas per pound of solidstreated.

The gases introduced into the hood 23 above the sinter burden forpassage through the burden are very moist. Contrary to the previousexperience, sogging out is not experienced. Perhaps this condition isalleviated by large gas flow. Hydrocarbons contained in the cold recycleare burned along with unburned carbonaceous material in the burden.Particulate material becomes entrained in the burden due to the fusedcondition of the burden at progressively deeper locations within thebody of the burden as the burden moves from right to left. The balanceof the moisture in the burden, i.e. approximately 50 to 10 percent, isalso removed in this region.

As indicated above, the sinter exhaust draft is drawn downwardly bymeans of the sinter fan 24 and optionally, though preferably, through adust collector 27 as also indicated in the drawing. These gases are ofsubstantially reduced hydrocarbon content and increased temperature, andare oxygen-depleted. They may be conducted directly to an electrostaticprecipitator or, alternatively, are introduced into an afterburner, asshown in the drawing, to further condition the gas for discharge to theatmosphere. The afterburner oxidizes carbon monoxide, burns any residualhydrocarbons or entrained carbonaceous material, and oxidizes any loweroxides of sulfur or entrained elemental sulfur. Natural gas and primaryair may be introduced into the afterburner to aid combustion. Thetemperature of the gas entering the electrostatic precipitator or theafterburner is much higher, i.e. on the order of 400 to 400F. instead ofthe usual 250F. average obtained in the conventional process. Thus,where passage through an afterburner is contemplated, the amount of fueland temperature elevation (up to about l500F.) required to effectafterburning is reduced.

Upon leaving the afterburner, the gas enters a gas cooler which may bein the form of a scrubbing tower whereby water-soluble components whichwould otherwise be discharged into the atmosphere as pollutants areefficiently removed. Alternatively, and preferbly, the cooling apparatusmay involve heat transfer from the gas to the water through a barrier tomaintain the gas and the water entirely separate. In this waywatersoluble salts, for example, do not become cooling water pollutants,but are carried along in the gas stream for later removal in theelectrostatic precipitator. By means of the cooler, the excessively hightemperature of the gas exhausting from the afterburner is lowered to anoptimum point for particulate solid removal in an electrostaticprecipitator, i.e. to about 400 to 500F. instead of the usual 250F.average in conventional processes. The moisture content or humidity ofthe gases at this point is approximately 30 percent to 40 percentvolume. The elevated temperature and the higher moisture content areconditions of the gas most favorable to successful electrostaticprecipitation of entrained solid particulate material. The resistivityof the gas at these conditions is optimum. Thus, the gas is finallypassed through an electrostatic precipitator diagrammaticallyillustrated beyond the cooling apparatus for exhaust therethrough to theatmosphere. The electrostatic precipitation apparatus is of conventionaldesign and its operation is well known to those skilled in the art. Anafterburner is not-required if the exhaust draft is sufficientlyconditioned for electrostatic precipitation without an afterburner.Also, it is apparent that the exhaust draft may initially.;be charged tothe electrostatic precipitator and finally to an afterburner.

In order to further cool the burden just prior to discharge of thesinter cake, primary air may be' drawn downwardly through the burden bymeans of a primary air fan 28 cooperating with windbox 16. Conventionaldust-collecting means 29 may be desirably inserted in this line. Thisprimary air, now w armed by passage through the hot burden andcontaining a higher percentage of oxygen thant he sinter exhaust, may beintroduced in the afterburner along with natural gas if necessary toeffect combustion of combustible components, solids, or gases in thesinter exhaust stream.

Also as a modification, a longer cooling zone can be extended beyondwindbox l6 and some or all'of the draft, still containing somehydrocarbons, from windboxes 7 through 15 can be routed through thislonger cooling zone as indicated by thedotted line 31 inthe drawingthereby thoroughly burning the hydrocarbons therein by contact with .hotsinter before this draft is finally exhausted to the electrostatic.precipitator as hydrocarbon-depleted gas, as indicated by dotted line 32in the drawing- In this .case, the afterburner, cooler and electrostaticprecipitator may be employed in series in the same manner previouslydescribed, as indicated by the solid'line 33in the drawing, or theafterburner may be bypassed asindicat'ed by the dotted line 34 in thedrawing, or both the afterburner and cooled may be bypassed, asindicated by the line 32 in the drawing, as circumstances may warrant.

In a typical example for the system of FIGURE 1, the cold recycleamounts to 81,500 ACFM (actual cubic feet per minute) at a temperatureof 230F. It analyzes Water 7 .771 Oxygen 15.8% Carbon dioxide 4.1%Carbon monoxide 0.8%

The sinter exhaust amounts to 136,000 ACFM at a temperature of 530F. Thesinter exhaust analyzes as follows:

Water 14.3%

Oxygen 12.6% Carbon dioxide l4.l7r

Carbon monoxide 1.28%

The afterburner exhaust amounts to 318,000 ACFM at a temperatureof1'400F. These gases analyze:

A typical exhaust from the cooler amounts to 213,000 ACFM at atemperature of 450F. This gas analyzes:

Water 36.671 Oxygen 7 .671 Carbon dioxide 6.671 Carbon monoxide 0.0%

The primary air in atypical example has a volume of 38,800 ACFM andexhausts at a temperature of 690F. These gases analyze as:

Water 0.0%

Oxygen 20.3%

Carbon dioxide 3.5%

Carbon monoxide By recycling sinter gases according to this invention,the total throughput of gas relative to the prior sintering process isabout halved and the temperature of the exhaust gas nearly doubled, i.e.up to as much as 550F. Thus, since the initial moisture in the burden isentrained in approximately half the draft volume, the water content ofthe draft reaching the electrostatic precipitator according to thisinvention is virtually doubled. The effect a of this procedure is, asindicated above, to decrease the resistivity of the gas/dust particlemedium and render the exhaust gas more favorably conditioned fortreatment by electrostatic procedures.

This process accomplishes still another very desirable form of exhaustgas treatment. Current regulations are trending toward the direction ofprohibiting the exhausting of any hydrocarbon or carbon-containinggaseous material, e.g. carbon monoxide, from a sinter process. In orderto meet these requirements, it has been suggested that conventionalsintering processes include an incinerator or afterburner piece ofequipment to enable elevation of the temperature of the conventionalsinter exhaust from the usual 250F. to as high as l300 or 1400F. Thismust be coupled with the introduction of air to effect combustion ofthese combustible ingreclients. In the present process, however, sincethe gases have been recycled, approximately 50 90 percent of thecombustible materials will undergo incineration within the body of thesinter bed, i.e. in the regions spanned by windboxes 7 in the drawing.Therefore, there is a greatly reduced volume of these moieties to beincinerated by some form of aftertreatment. Moreover, the gases willexhaust at a temperature considerably higher than the usual 250F.average in prior processes, as well as in lesser volume, so that wherean afterburner is employed the cost of fuel to raise the gases toincineration temperatures will be considerably less, and the amount ofoxygen-containing gas required to effect combustion will also be greatlyreduced, which further conserves fuel and other costs.

The terminal windbox 16 or windboxes are for the most part performingprimarily the function of cooling the sinter material, and combustion ispractically completed ahead of this portion of the path which the burden traverses. Accordingly, the exhaust gases from this portion of thepath largely consist of heated air which can serve as primary air ofcombustion in an afterburner as previously described. This furtherconserves heat and effects economies in the system. Also, as previouslydescribed, the terminal windboxes can be used for further incineratingexhaust gases by passing them through the cooling zone to effectcombustion of the residual entrained hydrocarbons.

One of the truly surprising discoveries attendant the process of thepresent invention is that the recycle gases rich in hydrocarbons passingthrough the flame front, where the total time of passage is measured inmilliseconds achieves substantially complete combustion of the burnableor combustible content of the gases within the body of the burden. Werethese hydrocarbons to be treated in afterburner equipment, suchequipment would have to be designed to a retention time of approximately1 to 5 seconds in order to achieve complete combustion. It is believedthat the presence of freshly formed metallic oxides in the sinter bedcatalyzes the oxidation of the hydrocarbons in the gas stream. I

The significantly reduced concentration of hydrocarbons in the sinterexhaust gases attendant applicants process as compared to conventionalsintering processes is demonstrated by the following: A sinter burdenwas prepared having the following composition on a natural basis, i.e.including water: classifier iron ore, 65 percent; manganese ore, 1percent; limestone, 7 percent; dolomite, 7 percent; flue dust andsludge, 5 percent; coke breeze, 5 percent; and scale, 10 percent. Theburden was sintered on a conventional Dwight- Lloyd sintering machine,having eight windboxes for providing draft.

In a first run no recycling was performed and exhaust gases from alleight windboxes were delivered to a main exhaust duct and the compositegas analyzed. The concentration of hydrocarbons in the analyzed gas wassuch that about 13.2 lbs. of hydrocarbons were discharged per hour.

In a second run the exhaust gases from the first three windboxesadjacent the feed end of the sintering machine were collected andrecycled downwardly through the burden passing over the next fourwindboxes. The other operating conditions of both runs were essentiallythe same. However, the concentration of hydrocarbons in the compositegas exhausting from those four windboxes and the last windbox (to whichgas was not recycled) was such that only about 3.0 lbs. of hydrocarbonswere discharged per hour. This represents a decrease in hydrocarbondischarge of about 77 percent, achieved with asingle recycling phase. Itis clear that these phases may be multiplied to recycle more frequentlyfrom one portion of the bed to a subsequent portion or portions as amultiphase system.

In the previous discussion, reference has been had generally toelectrostatic precipitators. While the process may be practiced withboth of the known types of electrostatic precipitators, i.e. the wet anddry types,

the process is most beneficial in conjunction with the dry-typeelectrostatic precipitators. Here, hot humid draft having in excess ofabout 10 percent humidity and a temperature in excess of about 400F. isproduced by the process, which conditions are most favorable for dryelectrostatic precipitation. The particulate material entrained in thedraft comprises not only particles of the ore and sinter material whichhave been mechanically entrained, but also particles of volatile metaland particles of metal salts, usually water soluble. These metal saltsare preferably precipitated by the dry electrostatic precipitatorsystems. In the latter, accumulations of particulate material upon thecharged plates are removed by vibratory means. The high humidity andhigh temperature of the gas being treated aids in the conductivity ofbuild-up of particulate material, this improving the operation of theprecipitator. However, because of the high temperature, the particlesare recovered in relatively dry form and may be disposed of withoutcontamination of aqueous washing media such as are employed in wetelectrostatic precipitation. The dry material recovered from theelectrostatic precipitator may be sold, recycled or buried for disposalpurposes.

Electrostatic precipitation, as is well known, contemplates passingparticle laden gas through a highly charged electrostatic field betweenmetallic conductors. The particles move toward a plate of oppositecharge and are discharged on the surface thereof. As

the layer of discharged particles builds up, efficiency begins todecline and the particles must be removed periodically. This may be done,by vibrators which shake the plates to dislodge accumulated particles,or by water washing. The former is the dry process, and the latter, thewet process as above mentioned. It has been found that with burdenscontaining high concentrations of flux materials to 30 percent) sharp,heavy blows are more efficient in cleaning the electrodes and platesthan high frequency rapping.

What is claimed is:

1. In a process for sintering a bed of particulate mineral materialmixed with combustible carbonaceous material and water while moving thebed on a traveling grate and while burning the carbonaceous material inthe bed to supply heat thereto, wherein carbonaceous material in the bedadjacent one surface thereof is ignited during a first portion of itstravel and combustion thereof progressively through the bed isthereafter maintained during its continued travel through an elongatecombustion zone by passing oxidizing gases into the bed over the lengthof said combustion zone and exhausting from the bed the gaseous productsof combustion of said carbonaceous material together with entrainedcombustible matter derived from the bed that includes a substantialamount of hydrocarbons, the exhaust gases from an initial portion of thelength of said combustion zone also entraining and removing a majorportion of the initial water content of the bed; the improvementcomprising collecting the exhaust gases and entrained matter exhaustedfrom the bed along said initial portion of said combustion zone andrecycling the same into the bed along a subsequent portion of saidcombustion zone to incinerate the entrained combustible matter withinthe bed while withdrawing exhaust gases from the bed along saidsubsequent portion of the combustion zone, whereby the latter exhaustgases from the subsequent portion of said combustion zone have asubstantially reduced content of combustible matter and a substantiallyincreased moisture content.

2. An improved sintering process in accordance with claim 1 in which thegas flow through the bed over the length of said combustion zone isdownward.

3. An improved sintering process in accordance with claim 1 in which theoxidizing gases and recycled gases are passed downwardly into the bedfrom the upper side thereof.

4. An improved sintering process in accordance with claim 1 in whichsaid latter exhaust gases are passed through an afterburner toincinerate and further reduce their content of combustible matter.

5. An improved sintering process in accordance with claim 1 in whichsaid latter exhaust gases are cooled to condition them for electrostaticprecipitation of particulate matter entrained therein.

6. An improved sintering process in accordance with claim 1 in whichsaid latter exhaust gases are passed through an afterburner toincinerate and further reduce their content of combustible matter andthen through a cooler to condition them for electrostatic precipitationof particulate matter entrained therein.

7. An improved sintering process in accordance with claim 6 in which airis passed through the bed along a terminal portion of its travel to coolthe sintered bed and preheat the air, and a portion of the thuspreheated air is introduced into said afterburner'to support thecombustion of combustible matter therein.

8. An improved sintering process in accordance with claim 6 in which airis passed through the bed along a terminal portion of its path of travelto cool the bed and preheat the air, and the preheated air is introducedinto said afterburner to support the combustion of combustible mattertherein.

9. An improved sintering process in accordance with claim 1 wherein thegas flow through the bed over the length of said combustion zone isdownwardly there.- through from the upper surface thereof, said latterexhaust gases are passed through an afterburner to incinerate andfurther reduce their content of combustible matter, and gas exiting fromthe afterburner is cooled to further condition it for electrostaticprecipitation of particulate matter entrained therein.

10. An improved sintering process in accordance with claim 9 in whichair is passed through the bed along a terminal portion of its path oftravel to cool the bed and preheat the air, and the preheated air isintroduced into said afterburner to support the combustion ofcombustible matter therein.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO. 9,189

DATED I September 50, 1975 INVENTOR(S) 1 Thomas E, Ban

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown betow:

Column 3, Line 1, after "through" change "a" to the Column 5, Line 48,after of the change "process" to present Column 6, Line 50, change"latter" to depth Column 7', Line #7, change tOOF. to 600%. ---7 Line 56change "preferbly" to preferably Column 8, Line 5, after "percent"insert by Line 51, the phrase 'Istill containing some hydrocarbons,"

should appear after "15" in line 52;

Line 45, change "cooled" to cooler Column 9, Line 29, after half theinsert former Column 11, Line 8, change "this" to thus i Signed andScaled this twenty-third Day Of December 1975 [SEAL] A ttest:

RUTH C. MASON Arresting Officer C. MARSHALL DANN Commissioner ofParentsand Trademarks

1. IN A PROCESS FOR SINTERING A BED OF PARTICULATE MINERAL MATERIALMIXED WITH COMBUSTIBLE CARBONACEOUS MATERIAL AND WATER WHILE MOVING THEBED ON A TRAVELING GRATE AND WHILE BURNING THE CARBONACEOUS METERIAL INTHE BED TO SUPPLY HEAT THERETO, WHEREIN CARBONACEOUS MEATERIAL IN THEBED ADJACENT ONE SURFACE THEREOF IS IGNITED DURING A FIRST PORTION OFITS TRAV EL AND COMUSTION THEREOF PROGRESSIVLY THROUGH THE BED IS ANDCOMBUSTION THEREOF PROGRESSIVELY THROUGH THE BED IS THEREAFTERMAINTAINED DURING ITS CONTINUED TRAVEL THROUGH AN ELONGATE COMBUSTIONZONE BY PASSING OXIDIZING GASSES INTO THE BED OVER THE LENGTH OF SAIDCONBUSTION ZONE AND EXHAUSTING FROM THE BED THE GASSEOUS PRODUCTS OFCOMBUSTION OF SAID CARBONACEOUS MATERIAL TOGETHER WITH ENTRAINEDCOMBUSTIBLE MATTER DERIVED FROM THE BED THAT INCLUDES A SUBSTANTIALAMOUNT OF HYDROCARBONS, THE EXHAUST GASSES FROM AN INITIAL PORTION OFTHE LENGTH OF SAID COMBUSTION ZONE ALSO ENTRAINING AND REMOVING A MAJORPORTION OF THE INITIAL WATER CONTENT OF THE BED: THE IMPROVEMENTCOMPRISING COLLECTING THE EXHAUST GASSES AND ENTRAINED MATTER EXHAUSTEDFROM THE BED ALONG SAID INITIAL PORTION OF SAID COMBUSTION ZONE ANDRECYCLING THE SAME INTO THE BED ALONG A SUBSEQUENT PORTION OF SAIDCOMBUSTION ZONE TO INCINERATE THE ENTRAINED COMBUSTIBLE MATTER WITHINTHE BED WHILE WITHDRAWING EXHAUST GASSES FROM THE BED ALONG SAIDSUBSEQUENT PORTION OF THE COMUSTION ZONE, WHEREBY THE LATTER EXHAUSTGASSES FROM THE SUBSEQUENT PORTION OF SAID COMBUSTION ZONE HAVE ASUBSTANTIALLY REDUCED CONTENT OF COMBUSTIBLE MATTER AND A SUBSTANTIALLYINCREASED MOISTURE CONTENT.
 2. An improved sintering process inaccordance with claim 1 in which the gas flow through the bed over thelength of said combustion zone is downward.
 3. An improved sinteringprocess in accordance with claim 1 in which the oxidizing gases andrecycled gases are passed downwardly into the bed from the upper sidethereof.
 4. An improved sintering process in accordance with claim 1 inwhich said latter exhaust gases are passed through an afterburner toincinerate and further reduce their content of combustible matter.
 5. Animproved sintering process in accordance with claim 1 in which saidlatter exhaust gases are cooled to condition them for electrOstaticprecipitation of particulate matter entrained therein.
 6. An improvedsintering process in accordance with claim 1 in which said latterexhaust gases are passed through an afterburner to incinerate andfurther reduce their content of combustible matter and then through acooler to condition them for electrostatic precipitation of particulatematter entrained therein.
 7. An improved sintering process in accordancewith claim 6 in which air is passed through the bed along a terminalportion of its travel to cool the sintered bed and preheat the air, anda portion of the thus preheated air is introduced into said afterburnerto support the combustion of combustible matter therein.
 8. An improvedsintering process in accordance with claim 6 in which air is passedthrough the bed along a terminal portion of its path of travel to coolthe bed and preheat the air, and the preheated air is introduced intosaid afterburner to support the combustion of combustible mattertherein.
 9. An improved sintering process in accordance with claim 1wherein the gas flow through the bed over the length of said combustionzone is downwardly therethrough from the upper surface thereof, saidlatter exhaust gases are passed through an afterburner to incinerate andfurther reduce their content of combustible matter, and gas exiting fromthe afterburner is cooled to further condition it for electrostaticprecipitation of particulate matter entrained therein.
 10. An improvedsintering process in accordance with claim 9 in which air is passedthrough the bed along a terminal portion of its path of travel to coolthe bed and preheat the air, and the preheated air is introduced intosaid afterburner to support the combustion of combustible mattertherein.